There is a decrease in pregnancy rates following fresh IVF transfers compared to transfers of frozen embryos in a natural cycle. It has been proposed that this decrease in uterine receptivity is due to alterations in the endometrium and we have evidence from a rat ovarian hyperstimulation (OH) model to support this.
At the time of receptivity during normal pregnancy there are distinct changes in uterine epithelial cells including modifications to the lateral plasma membrane. These changes include morphological and biochemical alterations in tight junctions, loss of adherens junctions and a decrease in the number of morphological desmosomes down the lateral plasma membrane.
At the time of receptivity during OH compared to normal pregnancy we found an increase in the number of morphological desmosomes and a corresponding increase is desmoglein-2 (a desmosomal marker) down the length of the lateral plasma membrane. We also observed similar changes in Rab 13, a protein involved in endosomal trafficking of proteins to the lateral plasma membrane, spreading down the entire length of the lateral plasma membrane at the time of receptivity during OH pregnancy.
These results add to the developing picture of the alterations in morphological and molecular markers of receptive uterine epithelial cells as a result of the effects of IVF medications on the endometrium, explaining the decrease in uterine receptivity during fresh stimulated IVF cycles.